FIG. 1 is a schematic block diagram of prior art system 10 of generating a pixel clock for a semiconductor inspection system. On known platforms used in the inspection of photo-masks with a normally incident chief ray, there is no coupling between the lateral (X direction) and vertical (Z direction) degrees of freedom. System 10 includes interpolating encoder 12 and phase lock loop 14 with phase detector 16 and voltage controlled oscillator (VCO) 18. Divider 20 is in feedback loop 22. Frequency control and phase accumulator circuit 24 combines signal 26 with signal 28 to generate pixel clock 30 used to control a transfer of charge in a time delay integration (TDI) charge-coupled device (CCD). Loop 14 synchronizes the pixel clock signal to the “varying” lateral velocity of the imaged pixels on the stage.
In addition to varying the pixel clock frequency with stage velocity, block 32 corrects, using map 34, for non-linearities in an X direction stage servo resulting from imperfect encoders, granite maps etc. Operation of system 10 is accomplished in a two stage process. First, the output of the VCO is generated. Then, the output of the VCO clocks circuit 24, which generates the pixel or line clock.
FIG. 2 is a schematic representation of known semiconductor inspection system 100 using off-axis illumination. Because no optical materials are transparent for extreme ultra-violet EUV, off-axis illumination must be used for EUV mask inspection, for example, of a multi-layer mask. For example, EUV source 102 transmits EUV chief ray 104 to surface 106 of photo-mask 108 at angle of illumination θ. Ray 104 reflects off of surface 104 at angle θ to TDI CCD 112, which transfers charges to generate and transmit data to processor 114 for generation of pixel images of the areas of surface 106 illuminated by ray 104. Typical angles of illumination are on the order of 6 to 8 degrees.
The use of ray 104 leads to a trigonometric coupling between vertical (X direction) and lateral (Z direction) motions of stage 116 holding the photo-mask for inspection. For example, the coupling results in apparent lateral position 118 for an imaged pixel that is displaced by amount δx (lateral error motion) from actual lateral position 120 for the pixel. The Z motion can result from a number of sources such as the surface map of the photo-mask and error motions in a Z direction stage servo due to the disturbance forces. The lateral error motion is significant enough to cause significant blur in the pixel images. Thus, the coupling described above poses problems with known methods of synchronizing photo-mask stage motions to the movement of charges across a TDI CCD. For example, system 10 is unable to address or provide a solution to the lateral error motion.